Golf ball

ABSTRACT

The invention provides a golf ball having a solid core, a cover of at least one layer, and a plurality of dimples on a surface of an outermost layer of the cover. The dimples number at least 250 but not more than 500, and have a volume ratio (VR) of from 1.20 to 1.60%. The outermost cover layer has a surface hardness, expressed as the Shore D hardness, of from 58 to 75. The ball has a deflection, when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), of from 4.0 to 6.0 mm. This golf ball is able to substantially reduce the distance traveled by the ball when struck at a high head speed, while at the same time holding down the decrease in distance when struck at a low head speed.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball which is composed of asolid core and a cover of at least one layer, and which has a pluralityof dimples on a surface of the outermost layer of the cover. Morespecifically, the invention relates to a golf ball which substantiallyreduces the distance traveled by the ball when struck at a high headspeed (head speed is sometimes abbreviated below as “HS”) while at thesame time minimizing the degree of reduction in distance when struck ata low head speed.

With recent advances in golfing equipment such as balls and clubs, golfballs have come to travel increasing distances. For this reason, to keepplay fair, strict rules have been adopted which establish, in the caseof a golf club, for example, the size of the head and the length of theshaft. Similarly, limitations have been placed on certaincharacteristics of the golf ball, such as its size, weight and initialvelocity, so as to restrict excessive ball travel of the sort that wouldresult in a loss of fair play.

The trend toward regulation is accelerating for golf balls as well, andthere is a possibility that the upper limit in the distance traveled bya golf ball under the conditions of use by a professional golfer, i.e.,under high HS conditions, will be further restricted. Generally, if thedistance a ball travels when played under high HS conditions is reduced,the distance traveled by the ball when played under the conditions ofuse by an ordinary amateur player, i.e., under low HS conditions, alsoends up decreasing to a similar degree. Accordingly, there exists adesire to satisfy the above restriction while at the same timeminimizing the decrease in distance by the golf ball when used by anordinary amateur player.

The distance traveled by a golf ball is generally held down by limitingthe initial velocity. However, in such cases, the distance traveleddecreases in about the same ratio both at high head speeds and low headspeeds. As a result, such balls have significant drawbacks for low HSplayers.

As another approach, a variety of golf balls have been disclosed which,by optimizing the dimples on the surface of the ball, lower the flighttrajectory and hold down the decrease in distance.

For example, JP-A 05-103846 describes a golf ball in which the dimplediameter, dimple depth and number of dimples have been optimized. JP-A10-043342 and JP-A 10-043343 disclose golf balls in which the amount ofdeformation by a ball when compressed under a load of 100 kgf has beenoptimized, along with which the dimple diameter divided by the dimpledepth has been set to from 10 to 15 or the dimple space volume as aproportion of the total volume of a hypothetical sphere were the surfaceof the ball to have no dimples thereon has been set to from 0.7 to 1.1%.JP-A 2000-107338 discloses a practice golf ball having an optimized ballweight and diameter.

It is therefore an object of the present invention to provide a golfball which can achieve a superior distance in a low HS range whileholding down the distance traveled in a high HS range.

SUMMARY OF THE INVENTION

The inventors have conducted extensive investigations in order toachieve the above object. As a result, they have found that, in a golfball which is composed of a solid core and a cover of at least onelayer, and which has a plurality of dimples on a surface of theoutermost layer of the cover, by specifying, for the dimples formed onthe surface of the outermost cover layer, the number of dimples and thedimple volume ratio (VR), and by optimizing the surface hardness of theoutermost cover layer and the deflection of the ball as a whole, thedistance traveled by the ball when struck at a high head speed can besubstantially reduced while at the same time holding down the decreasein distance when the ball is struck at a low head speed.

That is, unlike conventional methods which lower the ball initialvelocity or the core initial velocity, the golf ball of the presentinvention is able, by combining low-trajectory dimples with the internalstructure of the ball, to substantially reduce the distance traveled bythe ball when struck at a high head speed while at the same time holdingdown as much as possible, relative to the reduction in distance at highhead speed, the decrease in the distance traveled by the ball whenstruck at a low head speed. As used herein, “distance” refers to thetotal distance traveled by a golf ball, including both the carry and therun.

Accordingly, the invention provides the following golf balls.

[1] A golf ball comprising a solid core, a cover of at least one layer,and a plurality of dimples on a surface of an outermost layer of thecover, wherein the dimples number at least 250 but not more than 500 andhave a volume ratio (VR) of from 1.20 to 1.60%, the outermost coverlayer has a surface hardness, expressed as the Shore D hardness, of from58 to 75, and the ball has a deflection, when compressed under a finalload of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf),of from 4.0 to 6.0 mm.[2] The golf ball of [1], wherein the ball, under the conditions of ahead speed of 54 m/s, a ball initial velocity of 78.0±0.5 m/s, a launchangle of 9.7±0.5° and an initial backspin rate of 2,700±100 rpm, has atotal distance of not more than 290 yards.[3] The golf ball of [1], wherein the ball has a ratio of total distancetraveled when struck at a head speed of 54 m/s to total distancetraveled when struck at a head speed of 35 m/s (HS54/HS35) of from 1.30to 1.50.[4] The golf ball of [1], wherein the dimple volume ratio and thedeflection satisfy the relationship expressed by the formula:

4×VR+deflection=9.0 to 11.0.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a cross-sectional view showing an example of the internalstructure of the golf ball according to the present invention.

FIG. 2 is a schematic view illustrating a dimple used in the presentinvention.

FIG. 3 is a top view showing a dimple pattern (I) used on golf balls inexamples of the invention and comparative examples.

FIG. 4 is a top view showing a dimple pattern (II) used on a golf ballin an example of the invention.

FIG. 5 is a top view showing a dimple pattern (III) used on a golf ballin a comparative example.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.

The golf ball of the invention is a multi solid golf ball having a solidcore (referred to below as simply the “core”) and a cover of at leastone layer. A plurality of dimples are formed on a surface of anoutermost layer of the cover. By optimizing within specific ranges thesurface hardness of the outermost cover layer and the deflection of theball as a whole and combining therewith dimples which satisfy thesubsequently described specific parameters, the distance traveled by theball when struck at a high head speed can be substantially reduced whileholding down the decrease in the distance traveled by the ball whenstruck at a low head speed. As used in the present invention, “high HSrange” refers to a range of about 50 to 60 m/s and “low HS range” refersto a range of 30 to 40 m/s.

The internal structure of the golf ball G of the present invention needonly have a core and a cover of at least one layer, and may be suitablyset without particular limitation within a range that does not departfrom the objects of the invention. For example, when the ball is athree-piece solid golf ball having a two-layer cover composed of aninner layer and an outer layer, as shown in FIG. 1, the ball has athree-layer construction with at least a core 1, an inner cover layer 2which encases the core 1, and an outer cover layer 3 which encases theinner cover layer 2. When the ball has a multi-layer construction with acover of two or more layers, the cover layers are sometimes referred tocollectively herein as the “cover.” That is, in the case of thethree-piece solid golf ball shown in FIG. 1, the inner cover layer 2 andthe outer cover layer 3 are sometimes referred to collectively as the“cover.” A plurality of dimples D are generally formed on the surface ofthe outer cover layer 3, and these dimples D satisfy the subsequentlydescribed parameters of the invention. It should be noted that, althoughFIG. 1 shows a three-layer construction arrived at by forming a core, aninner cover layer 2 and an outer cover layer 3, as mentioned above, thisarrangement may be suitably modified within a range that does not departfrom the objects of the invention. For example, if necessary, the covermay be formed of one layer or of three or more layers. Or the core 1 maybe formed of a plurality of layers.

The core in the invention may be formed using a rubber compositioncontaining, for example, a base rubber and also such ingredients as aco-crosslinking agent, an organic peroxide, an inert filler, sulfur andan organosulfur compound. The base rubber of the rubber composition ispreferably one composed primarily of a known polybutadiene.

In the present invention, an organosulfur compound may be optionallyincluded in the base rubber in order to increase the rebound of thecore. When an organosulfur compound is included, the amount oforganosulfur compound per 100 parts by weight of the base rubber may beset to preferably at least 0.05 part by weight, more preferably at least0.1 part by weight, and even more preferably at least 0.2 part byweight. The upper limit in the amount included may be set to preferablynot more than 5 parts by weight, more preferably not more than 4 partsby weight by weight, and even more preferably not more than 2 parts byweight. If the amount of organosulfur compound included is too small, asufficient core rebound-increasing effect may not be obtained. On theother hand, if too much organosulfur compound is included, the core maybecome too soft, resulting in a poor feel when the ball is played and apoor durability to cracking on repeated impact.

The diameter of the core, although not subject to any particularlimitation, may be set to from 30 to 42 mm. In this case, the lowerlimit value is preferably at least 32 mm, more preferably at least 34mm, and even more preferably at least 35 mm. The upper limit value maybe set to preferably not more than 41 mm, more preferably not more than40 mm, even more preferably not more than 39 mm, and most preferably notmore than 38 mm.

The core deflection, i.e., the amount of deflection when compressedunder a final load of 1,275 N (130 kgf) from an initial load state of 98N (10 kgf), although not subject to any particular limitation, may beset within a range of from 3.0 to 9.0 mm. In this case, the lower limitvalue is preferably at least 3.5 mm, more preferably at least 4.0 mm,and even more preferably at least 4.5 mm. The upper limit value may beset to preferably not more than 8.0 mm, and more preferably not morethan 7.0 mm. If the core is too much harder than the above range (smalldeflection), a sufficient distance-reducing effect may not be achievableon shots taken at a high head speed. On the other hand, if the core istoo much softer than the above range (large deflection), the feel of theball may become too soft and the durability to cracking on repeatedimpact may worsen.

The specific gravity of the core, while not subject to any particularlimitation, may be set within a range of from 0.9 to 1.4. In such acase, the lower limit value is preferably at least 1.0, and morepreferably at least 1.1. The upper limit value may be set to preferablynot more than 1.3, and more preferably not more than 1.2.

In the present invention, by using the above material to form the solidcore 1, the rebound can be increased, thus enabling a golf ball capableof achieving a stable trajectory to be provided.

The golf ball G of the present invention has a cover of at least onelayer formed over the solid core 1. The number, material hardness (ShoreD) and thickness of the cover layers formed in this invention are notsubject to any particular limitation, and may be set as appropriatewithin ranges that do not depart from the objects of the invention. Forexample, when the three-piece solid golf ball shown in FIG. 1 ismanufactured by forming over the core 1 a two-layer cover composed of aninner cover layer 2 and an outer cover layer 3, these parameters may beset as indicated below. Here, “material hardness (Shore D)” refers tothe hardness of a sheet of the cover material that has been formed underapplied pressure to a thickness of about 2 mm, as measured using a typeD durometer in general accordance with ASTM D2240.

First, the material hardness (Shore D) of the inner cover layer,although not subject to any particular limitation, may be set to atleast 30, preferably at least 35, more preferably at least 40, and mostpreferably at least 45. It is recommended that the upper limit be notmore than 66, preferably not more than 63, and more preferably not morethan 60. When the material hardness (Shore D) of the inner cover layeris too high, the ball may have a poor feel on impact.

The thickness of the inner cover layer, although not subject to anyparticular limitation, may be set to at least 0.5 mm, preferably atleast 0.7 mm, more preferably at least 1.0 mm, and still more preferablyat least 1.3 mm. It is recommended that the upper limit be not more than3.0 mm, preferably not more than 2.5 mm, even more preferably not morethan 2.3 mm, and most preferably not more than 2.2 mm. When the innercover layer is too thin, the durability may worsen; when it is toothick, the ball may have a poor feel on impact.

The material hardness (Shore D) of the outer cover layer, although notsubject to any particular limitation, may be set to at least 55,preferably at least 57, and more preferably at least 59. It isrecommended that the upper limit be not more than 70, preferably notmore than 67, and more preferably not more than 64. If the materialhardness (Shore D) of the outer cover layer is too low, the feel onimpact may be too soft. On the other hand, if it is too high, thedurability or the feel on impact may worsen.

The thickness of the outer cover layer, although not subject to anyparticular limitation, may be set to at least 0.5 mm, preferably atleast 0.7 mm, and more preferably at least 0.8 mm. It is recommendedthat the upper limit be not more than 3.0 mm, preferably not more than2.5 mm, more preferably not more than 2.0 mm, and even preferably notmore than 1.6 mm. If the thickness of the outer cover layer fallsoutside the above range, this may lead to a worsening in the feel of theball on impact or in the durability.

In the present invention, the cover may be formed of a known material,examples of which include, but are not limited to, thermoplastic resinssuch as ionomeric resins, and various types of thermoplastic elastomers.Examples of thermoplastic elastomers include polyester-basedthermoplastic elastomers, polyamide-based thermoplastic elastomers,polyurethane-based thermoplastic elastomers, olefin-based thermoplasticelastomers and styrene-based thermoplastic elastomers.

In the present invention, such cover materials are not subject to anyparticular limitation, although preferred use may be made of a covermaterial composed primarily of a material selected from the groupconsisting of the polyurethane materials (I), polyurethane materials(II) and ionomeric resin materials shown below. These materials,including molding methods for the same, are described in order below.

Polyurethane Material (I)

This material (I) is composed primarily of components A and B below:

(A) a thermoplastic polyurethane material,(B) an isocyanate mixture obtained by dispersing (B-1) an isocyanatecompound having as functional groups at least two isocyanate groups permolecule in (B-2) a thermoplastic resin that is substantiallynon-reactive with isocyanate.

Golf balls in which the cover has been formed of this material (I) canbe endowed with an excellent feel, controllability, cut resistance,scuff resistance and durability to cracking on repeated impact.

Next, each of the above components is described.

The thermoplastic polyurethane material (A) has a structure whichincludes soft segments made of a polymeric polyol (polymeric glycol),and hard segments made of a chain extender and a diisocyanate. Here, thepolymeric polyol serving as a starting material is not subject to anyparticular limitation, and may be any that has hitherto been used in theart relating to thermoplastic polyurethane materials, such as polyesterpolyols and polyether polyols. Polyether polyols are preferable topolyester polyols because they enable the synthesis of thermoplasticpolyurethane materials having a high rebound resilience and excellentlow-temperature properties. Illustrative examples of polyether polyolsinclude polytetramethylene glycol and polypropylene glycol. From thestandpoint of rebound resilience and low-temperature properties,polytetramethylene glycol is especially preferred. The polymeric polyolhas an average molecular weight of preferably from 1,000 to 5,000. Amolecular weight of from 2,000 to 4,000 is especially preferred forsynthesizing thermoplastic polyurethane materials having a high reboundresilience.

The chain extender employed is preferably one that has hitherto beenused in the art relating to thermoplastic polyurethane materials.Illustrative, non-limiting, examples include 1,4-butylene glycol,1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and2,2-dimethyl-1,3-propanediol. These chain extenders have an averagemolecular weight of preferably from 20 to 15,000.

The diisocyanate employed is preferably one that has hitherto been usedin the art relating to thermoplastic polyurethane materials.Illustrative, non-limiting, examples include aromatic diisocyanates suchas 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and2,6-toluene diisocyanate; and aliphatic diisocyanates such ashexamethylene diisocyanate. However, depending on the type ofisocyanate, the crosslinking reaction during injection molding may bedifficult to control. In the practice of the invention, for stablereactivity with the subsequently described isocyanate mixture (B), it ismost preferable to use the following aromatic diisocyanate:4,4′-diphenylmethane diisocyanate.

A commercial product may be advantageously used as the thermoplasticpolyurethane material composed of the above-described material.Illustrative examples include those available under the trade namesPandex T-8290, Pandex T-8295 and Pandex T8260 (DIC Bayer Polymer, Ltd.),and those available under the trade names Resamine 2593 and Resamine2597 (Dainichi Seika Colour & Chemicals Mfg. Co., Ltd.).

The isocyanate mixture (B) is obtained by dispersing (B-1) an isocyanatecompound having as functional groups at least two isocyanate groups permolecule in (B-2) a thermoplastic resin that is substantiallynon-reactive with isocyanate. Here, the isocyanate compound (B-1) ispreferably an isocyanate compound that has hitherto been used in the artrelating to thermoplastic polyurethane materials. Illustrative,non-limiting, examples include aromatic diisocyanates such as4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and2,6-toluene diisocyanate; and aliphatic diisocyanates such ashexamethylene diisocyanate. From the standpoint of reactivity and worksafety, the use of 4,4′-diphenylmethane diisocyanate is most preferred.

The thermoplastic resin (B-2) is preferably a resin having a low waterabsorption and excellent compatibility with thermoplastic polyurethanematerials. Illustrative examples of such resins include polystyreneresins, polyvinyl chloride resins, ABS resins, polycarbonate resins, andpolyester elastomers (e.g., polyether-ester block copolymers,polyester-ester block copolymers). From the standpoint of the reboundresilience and strength, the use of a polyester elastomer, particularlya polyether-ester block copolymer, is especially preferred.

In the isocyanate mixture (B), it is desirable for the relativeproportions of the thermoplastic resin (B-2) and the isocyanate compound(B-1), expressed as the weight ratio (B-2):(B-1), to be from 100:5 to100:100, and especially from 100:10 to 100:40. If the amount of theisocyanate compound (B-1) relative to the thermoplastic resin (B-2) istoo small, a greater amount of the isocyanate mixture (B) will have tobe added to achieve an amount of addition sufficient for thecrosslinking reaction with the thermoplastic polyurethane material (A).As a result, the thermoplastic resin (B-2) will exert a large influence,rendering the physical properties of the material inadequate. On theother hand, if the amount of the isocyanate compound (B-1) relative tothe thermoplastic resin (B-2) is too large, the isocyanate compound(B-1) may cause slippage to occur during mixing, making preparation ofthe isocyanate mixture (B) difficult.

The isocyanate mixture (B) may be obtained by, for example, adding theisocyanate compound (B-1) to the thermoplastic resin (B-2) andthoroughly working together these components at a temperature of from130 to 250° C. using mixing rolls or a Banbury mixer, then eitherpelletizing or cooling and subsequently grinding. A commercial productsuch as that available under the trade name Crossnate EM30 (DainichiSeika Colour & Chemicals Mfg. Co., Ltd.) may be suitably used as theisocyanate mixture (B).

The above material (I) is composed primarily of the thermoplasticpolyurethane material (A) and isocyanate mixture (B) described above. Inthis material (I), the isocyanate mixture (B) is included in an amount,per 100 parts by weight of the thermoplastic polyurethane material (A),of at least 1 part by weight, preferably at least 5 parts by weight, andmore preferably at least 10 parts by weight, but not more than 100 partsby weight, preferably not more than 50 parts by weight, and morepreferably not more than 30 parts by weight. If too little isocyanatemixture (B) is included relative to the thermoplastic polyurethanematerial (A), a sufficient crosslinking effect will not be achieved. Onthe other hand, if too much is included, this may lead to discolorationof the molded material by unreacted isocyanate, which is undesirable.

In addition to above components (A) and (B), another component (C),although not essential, may also be included in the material (I). Thisother component is exemplified by thermoplastic polymeric materialsother than thermoplastic polyurethane materials; illustrative examplesinclude polyester elastomers, polyamide elastomers, ionomeric resins,styrene block elastomers, polyethylene, and nylon resins.

When component (C) is included, the amount is not subject to anyparticular limitation and may be suitably selected as appropriate forsuch purposes as adjusting the hardness, improving the resilience,improving the flow properties, and improving the adhesion of the covermaterial. The amount of component (C) included per 100 parts by weightof component (A) is set to preferably at least 10 parts by weight, andthe upper limit is set to not more than 100 parts by weight, preferablynot more than 75 parts by weight, and more preferably not more than 50parts by weight. If necessary, various additives such as pigments,dispersants, antioxidants, light stabilizers, ultraviolet absorbers andparting agents may also be suitably included in the above material (I).

Formation of the cover using the above material (I) may be carried outby a known molding method. For example, the cover may be molded byadding the isocyanate mixture (B) to the thermoplastic polyurethanematerial (A) and dry mixing, feeding the resulting mixture to aninjection molding machine, and injecting the molten resin blend over thecore. In such a case, the molding temperature differs with the type ofthermoplastic polyurethane material (A), although molding is generallycarried out within a temperature range of 150 to 250° C.

Reactions and crosslinking which take place in the golf ball coverobtained as described above are believed to involve the reaction ofisocyanate groups with hydroxyl groups remaining in the thermoplasticpolyurethane material to form urethane bonds, or the creation of anallophanate or biuret crosslinked form via a reaction involving theaddition of isocyanate groups to urethane groups in the thermoplasticpolyurethane material. Although the crosslinking reaction has not yetproceeded to a sufficient degree immediately after injection molding ofthe material (I), the crosslinking reaction can be made to proceedfurther by carrying out an annealing step after molding, in this wayconferring the golf ball cover with useful characteristics. “Annealing,”as used herein, refers to heat aging the cover at a constant temperaturefor a fixed length of time, or aging the cover for a fixed period atroom temperature.

Polyurethane Material (II)

This material (II) is a single resin blend in which the primarycomponents are (D) a thermoplastic polyurethane and (E) a polyisocyanatecompound. By forming a cover composed primarily of such a polyurethanematerial (II), it is possible to achieve an excellent feel,controllability, cut resistance, scuff resistance and durability tocracking on repeated impact without a loss of resilience.

As used herein, reference to a “single” resin blend means that the resinblend is not fed as a plurality of types of pellets, but rather issupplied to, for example, an injection molding machine as one type ofpellet prepared by incorporating a plurality of ingredients intoindividual pellets.

To fully and effectively achieve the objects of the invention, anecessary and sufficient amount of unreacted isocyanate groups should bepresent within the cover resin material. Specifically, it is recommendedthat the combined weight of above components (D) and (E) account for atleast 60%, and preferably at least 70%, of the total weight of thecover. Components (D) and (E) are described in detail below.

The above thermoplastic polyurethane (D) is described. The thermoplasticpolyurethane structure includes soft segments made of a polymeric polyol(polymeric glycol) that is a long-chain polyol, and hard segments madeof a chain extender and a polyisocyanate compound. Here, the long-chainpolyol used as a starting material is not subject to any particularlimitation, and may be any that has hitherto been used in the artrelating to thermoplastic polyurethanes. Exemplary long-chain polyolsinclude polyester polyols, polyether polyols, polycarbonate polyols,polyester polycarbonate polyols, polyolefin polyols, conjugated dienepolymer-based polyols, castor oil-based polyols, silicone-based polyolsand vinyl polymer-based polyols. These long-chain polyols may be usedsingly or as combinations of two or more thereof. Of the long-chainpolyols mentioned here, polyether polyols are preferred because theyenable the synthesis of thermoplastic polyurethanes having a highrebound resilience and excellent low-temperature properties.

Illustrative examples of the above polyether polyol includepoly(ethylene glycol), poly(propylene glycol), poly(tetramethyleneglycol) and poly(methyltetramethylene glycol) obtained by thering-opening polymerization of cyclic ethers. These polyether polyolsmay be used singly or as a combination of two or more thereof. In thepresent invention, poly(tetramethylene glycol) andpoly(methyltetramethylene glycol) are preferred.

It is preferable for these long-chain polyols to have a number-averagemolecular weight in a range of 1,500 to 5,000. By using a long-chainpolyol having a number-average molecular weight within this range, golfballs made with a thermoplastic polyurethane composition havingexcellent properties such as resilience and manufacturability can bereliably obtained. The number-average molecular weight of the long-chainpolyol is more preferably in a range of 1,700 to 4,000, and even morepreferably in a range of 1,900 to 3,000.

As used herein, “number-average molecular weight of the long-chainpolyol” refers to the number-average molecular weight calculated basedon the hydroxyl number measured in accordance with JIS K-1557.

Any chain extender employed in the prior art relating to thermoplasticpolyurethane materials may be advantageously used as the chain extender.For example, low-molecular-weight compounds with a molecular weight of400 or less which have on the molecule two or more active hydrogen atomscapable of reacting with isocyanate groups are preferred. Illustrative,non-limiting, examples of the chain extender include 1,4-butyleneglycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and2,2-dimethyl-1,3-propanediol. In the present invention, an aliphaticdiol having 2 to 12 carbons is preferred, and 1,4-butylene glycol ismore preferred.

Any polyisocyanate compound hitherto employed in the art relating tothermoplastic polyurethane materials may be advantageously used withoutparticular limitation as the polyisocyanate compound. For example, usemay be made of one or more selected from the group consisting of4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluenediisocyanate, p-phenylene diisocyanate, xylylene diisocyanate,1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate,hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate,tetramethylene diisocyanate, hexamethylene diisocyanate, isophoronediisocyanate, norbornene diisocyanate, trimethylhexamethylenediisocyanate and dimer acid diisocyanate. However, depending on the typeof isocyanate, the crosslinking reaction during injection molding may bedifficult to control. In the practice of the invention, to provide abalance between stability at the time of production and the propertiesthat are manifested, it is most preferable to use 4,4′-diphenylmethanediisocyanate, which is an aromatic diisocyanate.

It is most preferable for the thermoplastic polyurethane serving asabove component D to be a thermoplastic polyurethane synthesized using apolyether polyol as the long-chain polyol, using an aliphatic diol asthe chain extender, and using an aromatic diisocyanate as thepolyisocyanate compound. It is desirable, though not essential, for thepolyether polyol to be polytetramethylene glycol having a number-averagemolecular weight of at least 1,900, for the chain extender to be1,4-butylene glycol, and for the polyisocyanate compound to be4,4′-diphenylmethane diisocyanate.

The mixing ratio of active hydrogen atoms to isocyanate groups in theabove polyurethane-forming reaction can be adjusted within a desirablerange so as to make it possible to obtain a golf ball which is composedof a thermoplastic polyurethane composition and has various improvedproperties, such as rebound, spin performance, scuff resistance andmanufacturability. Specifically, in preparing a thermoplasticpolyurethane by reacting the above long-chain polyol, polyisocyanatecompound and chain extender, it is desirable to use the respectivecomponents in proportions such that the amount of isocyanate groups onthe polyisocyanate compound per mole of active hydrogen atoms on thelong-chain polyol and the chain extender is from 0.95 to 1.05 moles.

No particular limitation is imposed on the method of preparing component(D). Production may be carried out by either a prepolymer process or aone-shot process in which the long-chain polyol, chain extender andpolyisocyanate compound are used and a known urethane-forming reactionis effected. Of these, a process in which melt polymerization is carriedout in a substantially solvent-free state is preferred. Production bycontinuous melt polymerization using a multiple screw extruder isespecially preferred.

A commercial product may be used as component (D). Illustrative examplesinclude products available under the trade names Pandex T8295, PandexT8290 and Pandex T8260 (DIC Bayer Polymer, Ltd.).

Next, concerning the polyisocyanate compound used as component E, it isessential that, in at least some portion thereof within a single resinblend, all the isocyanate groups on the molecule remain in an unreactedstate. That is, polyisocyanate compound in which all the isocyanategroups on the molecule are in a completely free state should be presentwithin a single resin blend, and such a polyisocyanate compound may bepresent together with a polyisocyanate compound in which a portion ofthe isocyanate groups on the molecule are in a free state.

Various isocyanates may be used without particular limitation as thepolyisocyanate compound. Specific examples include one or more selectedfrom the group consisting of 4,4′-diphenylmethane diisocyanate,2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylenediisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate,tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate,dicyclohexylmethane diisocyanate, tetramethylene diisocyanate,hexamethylene diisocyanate, isophorone diisocyanate, norbornenediisocyanate, trimethylhexamethylene diisocyanate and dimer aciddiisocyanate. Of the above group of isocyanates, using4,4′-diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate andisophorone diisocyanate is preferred for achieving a good balancebetween the influence on moldability by, for example, the rise inviscosity associated with reaction with the thermoplastic polyurethaneserving as component D, and the properties of the resulting golf ballcover material.

In material (II), although not an essential ingredient, a thermoplasticelastomer other than the above thermoplastic polyurethane may beincluded as component F in addition to above components D and E.Including this component F in the above resin blend enables the flowproperties of the resin blend to be further improved and enables variousproperties required of golf ball cover materials, such as resilience andscuff resistance, to be enhanced.

This component F, which is a thermoplastic elastomer other than theabove thermoplastic polyurethane, is exemplified by one or morethermoplastic elastomer selected from among polyester elastomers,polyamide elastomers, ionomeric resins, styrene block elastomers,hydrogenated styrene-butadiene rubbers,styrene-ethylene/butylene-ethylene block copolymers and modified formsthereof, ethylene-ethylene/butylene-ethylene block copolymers andmodified forms thereof, styrene-ethylene/butylene-styrene blockcopolymers and modified forms thereof, ABS resins, polyacetals,polyethylenes and nylon resins. The use of polyester elastomers,polyamide elastomers and polyacetals is especially preferred because theresilience and scuff resistance are enhanced, owing to reactions withisocyanate groups, while at the same time a good manufacturability isretained.

The relative proportions of above components D, E and F are not subjectto any particular limitation. However, to fully achieve the advantageouseffects of the invention, it is preferable for the weight ratio amongthe respective components to be (D):(E):(F)=100:2 to 50:0 to 50, andmore preferably (D):(E):(F)=100:2 to 30:8 to 50.

In this invention, a single resin blend for forming the cover isprepared by mixing together component D, component E, and also optionalcomponent F. At this time, it is essential to select the mixingconditions such that, of the polyisocyanate compound, at least somepolyisocyanate compound is present in which all the isocyanate groups onthe molecule remain in an unreacted state. For example, treatment suchas mixture in an inert gas (e.g., nitrogen) or in a vacuum state must befurnished. The resin blend is then injection-molded over a core whichhas been placed in a mold. To smoothly and easily handle the resinblend, it is preferable for the blend to be formed into pellets having alength of 1 to 10 mm and a diameter of 0.5 to 5 mm. Sufficientisocyanate groups in an unreacted state remain in these resin pellets;the unreacted isocyanate groups react with component D or component F toform a crosslinked material while the resin blend is beinginjection-molded about the core, or due to post-treatment such asannealing thereafter.

In addition, various optional additives may also be included in thiscover-forming resin blend. For example, pigments, dispersants,antioxidants, light stabilizers, ultraviolet absorbers, and partingagents may be suitably included.

The melt mass flow rate (MFR) of this resin blend at 210° C. is notsubject to any particular limitation. However, to increase the flowproperties and manufacturability, the MFR is preferably at least 5 g/10min, and more preferably at least 6 g/10 min. If the melt mass flow rateof the resin blend is too low, the flow properties will decrease, whichmay cause eccentricity during injection molding and may also lower thedegree of freedom in the thickness of the cover that can be molded. Themelt mass flow rate is a measured value obtained in accordance with JISK-7210 (1999 edition).

The method of molding the cover may involve feeding the above resinblend to an injection-molding machine and injecting the molten resinblend over the core. Although the molding temperature in this case willvary depending on the type of thermoplastic polyurethane, the moldingtemperature is generally from 150 to 250° C.

When injection molding is carried out, it is desirable though notessential to carry out molding in a low-humidity environment such as bypurging with an inert gas (e.g., nitrogen) or a low-moisture gas (e.g.,low dew-point dry air), or vacuum treating, some or all places on theresin paths from the resin feed area to the mold interior. Illustrative,non-limiting, examples of the medium used for transporting the resininclude low-moisture gases such as low dew-point dry air or nitrogen. Bycarrying out molding in such a low-humidity environment, reaction by theisocyanate groups is kept from proceeding before the resin has beencharged into the mold interior. As a result, polyisocyanate in which theisocyanate groups are present in an unreacted state is included to somedegree in the molded resin material, thus making it possible to reducevariable factors such as an unnecessary rise in viscosity and enablingthe real crosslinking efficiency to be enhanced.

Techniques that may be used to confirm the presence of polyisocyanatecompound in an unreacted state within the resin blend prior to injectionmolding about the core include those which involve extraction with asuitable solvent that selectively dissolves out only the polyisocyanatecompound. An example of a simple and convenient method is one in whichconfirmation is carried out by simultaneous thermogravimetric anddifferential thermal analysis (TG-DTA) measurement in an inertatmosphere. For example, when the above-described single resin blend(material (II)) is heated in a nitrogen atmosphere at a temperatureramp-up rate of 10° C./min, a gradual drop in the weight ofdiphenylmethane diisocyanate can be observed from about 150° C. On theother hand, in a resin sample in which the reaction between thethermoplastic polyurethane material and the isocyanate mixture has beencarried out to completion, a weight drop is not observed from about 150°C., but a weight drop can be observed from about 230 to 240° C.

After the above material (II) has been injection-molded to form a cover,the properties as a golf ball cover can be additionally improved bycarrying out annealing so as to induce the crosslinking reaction toproceed further. “Annealing,” as used herein, refers to aging the coverin a fixed environment for a fixed length of time.

Ionomeric Resin Material

In the present invention, “ionomeric resin material” refers to a resincomposition which is composed primarily of a metal salt of anolefin-unsaturated carboxylic acid-unsaturated carboxylic acid esterrandom copolymer and/or a metal salt of an olefin-unsaturated carboxylicacid random copolymer.

The olefin generally has a number of carbons that is at least 2, but notmore than 8, and preferably not more than 6. Illustrative examplesinclude ethylene, propylene, butene, pentene, hexene, heptene andoctene. Ethylene is especially preferred.

Illustrative examples of the unsaturated carboxylic acid include acrylicacid, methacrylic acid, maleic acid and to fumaric acid. Acrylic acidand methacrylic acid are especially preferred.

The unsaturated carboxylic acid ester may be, for example, a lower alkylester of an unsaturated carboxylic acid. Illustrative examples includemethyl methacrylate, ethyl methacrylate, propyl methacrylate, butylmethacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butylacrylate. The use of butyl acrylate (n-butyl acrylate, isobutylacrylate) is especially preferred.

The random copolymer may be obtained by the random copolymerization ofthe above ingredients in accordance with a known method. Here, theunsaturated carboxylic acid content (acid content) within the randomcopolymer, although not subject to any particular limitation, may be setto generally at least 2 wt %, preferably at least 6 wt %, and morepreferably at least 8 wt %, it is recommended that the upper limit inthe unsaturated carboxylic acid content (acid content), although notsubject to any particular limitation, be generally not more than 25 wt%, preferably not more than 20 wt %, and more preferably not more than15 wt %. At a low acid content, the rebound may decrease, whereas at ahigh acid content, the processability of the material may decrease.

Some or all of the acid groups in the random copolymer are neutralizedwith metal ions. Although the degree of neutralization in this case isnot subject to any particular limitation, it is recommended that atleast 20 mol %, preferably at least 30 mol %, more preferably at least40 mol %, and even more preferably at least 70 mol %, of the acid groupsbe neutralized. The upper limit in the degree of neutralization,although not subject to any particular limitation, may be set to 100 mol% or less, preferably 95 mol % or less, and more preferably 90 mol % orless. At a degree of neutralization below 20%, the rebound may decrease.Here, the metal ions which neutralize the acid groups are exemplified byNa⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ and Pb⁺⁺. In thepresent invention, of these, Na⁺, Li⁺, Zn⁺⁺, Mg⁺⁺ and Ca⁺⁺ areespecially preferred.

No particular limitation is imposed on the content of the above metalsalt of a random copolymer (ionomeric resin), although the metal salt ispreferably included in an amount of from 100 to 50 wt % based on theoverall resin composition. In this case, the lower limit is morepreferably at least 60 wt %, even more preferably at least 70 wt %, andmost preferably at least 80 wt %. The upper limit is more preferably 95wt % or less, and even more preferably 90 wt % or less. In thisinvention, a known material may be used as the ionomeric resin material.Specific examples include those available from E.I. DuPont de Nemours &Co. under the trade names HPF 1000 and HPF 2000, and the resincompositions mentioned in U.S. patent application Ser. No. 12/340,790(or U.S. patent application Ser. No. 12/706,175). These may be usedsingly or as mixtures of two or more thereof.

To further improve the feel of the ball on impact, various non-ionomericthermoplastic elastomers may also be included. Such non-ionomericthermoplastic elastomers are exemplified by olefin-based thermoplasticelastomers, styrene-based thermoplastic elastomers, ester-basedthermoplastic elastomers and urethane-based thermoplastic elastomers. Inthe present invention, the use of an olefin-based thermoplasticelastomer is especially preferred. The olefin-based thermoplasticelastomer is a thermoplastic block copolymer having a crystallinepolyolefin block and a polyethylene/butylene random copolymer. Thisolefin-based thermoplastic elastomer is exemplified by thermoplasticblock copolymers composed of a crystalline polyethylene block (E) as ahard segment and a block of a relatively random copolymer of ethyleneand butylene (EB) as a soft segment. Preferred use may be made of blockcopolymers having a molecular structure with a hard segment at one orboth ends, such as block copolymers having an E-EB or E-EB-E structure.

These may be obtained by, for example, hydrogenating a polybutadiene.Here, the polybutadiene used in hydrogenation is preferably one in whichbonding within the butadiene structure is characterized by a 1,4-bondcontent in the overall butadiene structure of from 95 to 100 wt %, andin which from 50 to 100 wt %, and preferably from 80 to 100 wt %, of the1,4-bonds are present as block-like regions.

The above-mentioned E-EB-E type thermoplastic block copolymer ispreferably one obtained by hydrogenating a polybutadiene having at bothends of the molecular chain 1,4-polymerization products which are richin 1,4-bonds and having an intermediate region where 1,4-bonds and1,2-bonds are intermingled. The degree of hydrogenation (conversion ofdouble bonds on the polybutadiene to saturated bonds) in thepolybutadiene hydrogenate is preferably from 60 to 100%, and morepreferably from 90 to 100%. Too low a degree of hydrogenation may giverise to undesirable effects such as gelation in the blending step withother components such as an ionomeric resin and, when the golf ball hasbeen formed, may lead to a poor durability to impact.

In the block copolymer having an E-EB or E-EB-E molecular structure witha hard segment at one or both ends that may be advantageously used asthe thermoplastic block copolymer, the content of the hard segments ispreferably from 10 to 50 wt %. If the hard segment content is too high,the cover may lack sufficient softness, making it difficult toeffectively achieve the objects of the invention. On the other hand, ifthe hard segment content is too low, the blend may have a poormoldability.

The thermoplastic block copolymer has a melt mass flow index, at a testtemperature of 230° C. and a test load of 21.2 N, of preferably from0.01 to 15 g/10 min, and more preferably from 0.03 to 10 g/10 min.Outside of this range, problems such as weld lines, sink marks and shortshots may arise during injection molding. Moreover, it is preferable forthe thermoplastic block copolymer to have a surface hardness of from 10to 50. If the surface hardness is too low, the golf ball may have adecreased durability to repeated impact. On the other hand, if thesurface hardness is too high, a blend of the thermoplastic blockcopolymer with an ionomeric resin may have a decreased rebound. Thethermoplastic block copolymer has a number-average molecular weight ofpreferably from 30,000 to 800,000.

A commercial product may be used as the olefin-based thermoplasticelastomer. Illustrative examples include those available under the tradenames Dynaron 6100P, Dynaron 6200P and Dynaron 6201B (JSR Corporation).Of these, Dynaron 6100P, which is a block polymer having crystallineolefin blocks at both ends, is especially preferred for use in thepresent invention. These olefinic thermoplastic elastomers may be usedsingly or as mixtures of two or more thereof.

Various additives may also be optionally included in the above resincomposition. Examples of additives which may be suitably included arepigments, dispersants, antioxidants, ultraviolet absorbers and opticalstabilizers.

The cover material used in the invention may be a known cover material.Although not subject to any particular limitation, preferred use may bemade of the above-described polyurethane material (I), polyurethanematerial (II) or ionomeric resin material.

In the golf ball of the invention, by specifying the surface hardness ofthe outermost cover layer and the deflection of the ball as a whole andalso forming dimples which satisfy the subsequently described specificparameters and are able to achieve a relatively low trajectory, it ispossible to greatly reduce the distance traveled by the golf ball onshots taken at a high head speed while also holding down the decrease indistance traveled by the ball on shots taken at a low head speed. Theparameters for the dimples formed on the inventive golf ball aredescribed in detail below.

In the present invention, dimples having the following parameters (1) to(3) are formed on the surface of the cover formed of the above-describedmaterial. In cases where the surface of the ball is subjected tofinishing treatment (e.g., painting and stamping) after the cover hasbeen formed, parameters (1) to (3) below are calculated based on theshape of the dimples on the finished ball in which such treatment hasbeen entirely completed.

Dimple Parameter (1)

The total number of dimples is set in a range of at least 250 but notmore than 500. In this case, the lower limit may be set to preferably atleast 280, more preferably at least 300, and even more preferably atleast 310. The upper limit may be set to preferably not more than 450,more preferably not more than 420, even more preferably not more than400, and most preferably not more than 350.

Dimple Parameter (2)

To improve aerodynamic performance, the dimple surface coverage (SR),defined as the sum of the surface areas on a hypothetical sphere thatare circumscribed by the edges of the respective dimples as a proportionof the surface area of the hypothetical sphere, while not subject to anyparticular limitation, is set to preferably at least 70%. The SR may beset to more preferably at least 71%, and even more preferably at least72%.

Dimple Parameter (3)

To improve the aerodynamic performance, the dimple to volume ratio (VR),defined as the sum of the volumes of individual dimple spaces below aflat plane circumscribed by the edge of each dimple on a golf ball as aproportion of the volume of the hypothetical sphere were the golf ballto have no dimples on the surface, is set to from 1.20 to 1.60%. Thelower limit is preferably at least 1.22%, and more preferably at least1.24%. The upper limit is preferably not more than 1.55%, morepreferably not more than 1.50%, and even more preferably not more than1.46%. In cases where the volume ratio is larger than the above range,the trajectory may become too low, as a result of which the ball may nottravel far enough on shots taken at a low head speed. On the other hand,when the volume ratio is smaller than the above range, a sufficientdistance-reducing effect may not be achieved on shots taken at a highhead speed.

The shapes of the dimples are not limited to circular shapes, and mayalso be suitably selected from among, for example, polygonal,tear-shaped and oval shapes. Setting the number of dimple types to atleast three, and preferably at least five, makes it possible for thedimples to cover a higher proportion of the spherical surface. Also, byinterspersing large and small dimples, the surface coverage can beincreased to the specified range. Because this enables extremefluctuations in the coefficient of lift (CL) within the low-velocityregion to be suppressed, the ball trajectory can be made relatively low,thus making it easier to elicit the advantageous effects of theinvention.

The golf ball of the invention can be made to conform with the Rules ofGolf for competitive play, and may be formed to a diameter of not lessthan 42.67 mm. It is suitable to set the weight to generally not lessthan 45.0 g, and preferably not less than 45.2 g, but not more than45.93 g.

The golf ball of the invention is composed of the above-described coreand the cover of at least one layer, and has a plurality of dimples on asurface of the outermost layer of the cover. The ball as a whole has adeflection, when compressed under a final load of 1,275 N (130 kgf) froman initial load state of 98 N (10 kgf), of at least 4.0 mm, preferablyat least 4.1 mm, more preferably at least 4.2 mm, and even morepreferably at least 4.3 mm. The deflection has an upper limit which isnot more than 6.0 mm, preferably not more than 5.8 mm, more preferablynot more than 5.6 mm, and even more preferably not more than 5.3 mm. Ifthe deflection is too small, the distance traveled by the ball on shotstaken a high head speed may be excessive, making it impossible toachieve the reduction in distance on high HS shots that is the object ofthe invention. On the other hand, if the deflection is too large, theball may have a poor durability to cracking and may have an excessivelysoft feel on impact.

It is critical for the surface hardness of the outermost cover layer(i.e., the surface hardness of the ball), expressed as the Shore Dhardness, to be at least 58, preferably at least 59, more preferably atleast 60, and even more preferably at least 61. The upper limit value isset to not more than 75, and may be set to preferably not more than 72,more preferably not more than 70, and even more preferably not more than68. A ball surface hardness which is too low may worsen the scuffresistance, and a ball surface hardness which is too high may worsen thedurability to cracking. As used herein, the ball surface hardness (ShoreD) is a value measured at a dimple-free land portion of the ball surfacewith a type D durometer in accordance with ASTM-2240.

The initial velocity of the ball, as measured using an initial velocitymeasuring apparatus of the same type as the USGA drum rotation-typeinitial velocity instrument, although not subject to any particularlimitation, is preferably from 72.0 to 77.7 m/s. The lower limit valueis more preferably at least 74.0 m/s, and even more preferably at least75.0 m/s. The upper limit value is more preferably not more than 77.6m/s, and even more preferably not more than 77.4 m/s.

In the golf ball of the invention, although not subject to anyparticular limitation, from the standpoint of suppressing the reductionin distance on shots taken at a low head speed while markedly reducingthe distance on shots taken at a high head speed, it is preferable forthe dimple volume ratio and the deflection described above to satisfythe relationship expressed by the formula:

4×VR+deflection=9.0 to 11.0.

When the relationship between the dimple volume ratio and the deflectiondoes not satisfy the above formula, a sufficient distance-reducingeffect may not be achieved on shots taken at a high head speed, inaddition to which a good feel and a sufficient durability to crackingmay not be attained. In the present invention, the above formula servesas an indicator representing a proper relationship between the dimplesand the ball construction that ensures the distance traveled by the ballwhen struck at a low head speed while suppressing the distance traveledwhen struck at a high head speed.

As explained above, the golf ball of the invention greatly reduces thedistance traveled by the ball on shots taken at a high head speed. Insuch cases, although not subject to any particular limitation, it ispreferable for the ball, under the conditions of a head speed of 54 m/s,a ball initial velocity of 78.0±0.5 m/s, a launch angle of 9.7±0.5° andan initial backspin rate of 2,700±100 rpm, to have a total distance ofnot more than 290 yards.

Also, it is preferable for the difference in total distance traveled bythe ball depending on the magnitude of the head speed to be small.Although not subject to any particular limitation, it is recommendedthat the ball have a ratio of total distance traveled when struck at ahead speed of 54 m/s to total distance traveled when struck at a headspeed of 35 m/s (HS54/HS35) of at least 1.30. The lower limit value inthis ratio is preferably at least 1.35, more preferably at least 1.38,and even more preferably at least 1.40. The upper limit value ispreferably not more than 1.49.

As described above, in this invention, it is possible to substantiallyreduce the distance traveled by the ball on high HS shots while at thesame time minimizing the decrease in distance traveled by the ball onlow HS shots. As a result, there can be obtained a superior golf ballfor competitors having a low head speed.

EXAMPLES

The following Examples and Comparative Examples are provided by way ofillustration and not by way of limitation.

Examples 1 to 5 Comparative Examples 1 to 5

The rubber compositions shown in Table 1 were prepared, then molded andvulcanized at 155° C. for 15 minutes to produce solid cores. Numbers inthe table indicate parts by weight.

TABLE 1 A B C D E F Polybutadiene 75 85 85 100 65 100 rubber Isoprenerubber 25 15 15 0 20 0 Butyl rubber 0 0 0 0 15 0 Peroxide 1 0.6 0.6 0.60.6 0.6 0.6 Peroxide 2 0.6 0.6 0.6 0.6 0.6 0.6 Barium sulfate 22.4 24.421.8 22.4 20.7 21.7 Zinc oxide 4 4 4 4 4 4 Antioxidant 0.1 0.1 0.1 0.10.1 0.1 Zinc diacrylate 20 15 20 20 24 20 Zinc distearate 5.0 5.0 5.05.0 5.0 5.0 Calcium 10 10 0 10 10 0 carbonate Zinc salt 0 0 0 0 0 0.1 ofpenta- chlorothio- phenol Specific gravity 1.205 1.205 1.162 1.205 1.2051.162

Trade names of the materials in the table are as follows.

-   Polybutadiene rubber: Available under the trade name “BR 01” from    JSR Corporation.-   Isoprene rubber: Available under the trade name “IR 2200” from JSR    Corporation.-   Butyl rubber: Available under the trade name “Bromobutyl 2222” from    Japan Butyl Co., Ltd.-   Peroxide 1: Dicumyl peroxide, available under the trade name    “Percumyl D” from NOF Corporation.-   Peroxide 2: 1,1-Bis(t-butylperoxy)cyclohexane, available under the    trade name “Perhexa C-40” from NOF Corporation.-   Barium sulfate: Available under the trade name “Precipitated Barium    Sulfate 100” from Sakai Chemical Industry Co., Ltd.-   Zinc oxide: Available from Sakai Chemical Industry Co., Ltd.-   Antioxidant: Available under the trade name “Nocrac NS-6” from Ouchi    Shinko Chemical Industry Co., Ltd.-   Zinc diacrylate: Available from Nihon Jyoryu Kogyo Co., Ltd.-   Zinc distearate: Available under the trade name “Zinc Stearate G”    from NOF Corporation.

Next, the cover material shown in Table 2 below was injection-moldedover the core, thereby obtaining a golf ball in which the core isencased within an inner cover layer and an outer cover layer of giventhicknesses.

TABLE 2 Cover material (pbw) (1) (2) (3) (4) HPF 2000 100 0 0 0 Himilan1605 0 50 0 0 Himilan 1706 0 50 0 0 Pandex T8290 0 0 0 75 Pandex T8295 00 100 25 Polyisocyanate compound 0 0 9 9 Thermoplastic elastomer 0 0 1515 Titanium oxide 0 2 3.5 3.5 Polyethylene wax 0 0 1.5 1.5

Trade names of the materials in the table are as follows.

-   HPF 2000: An ionomeric resin available from E.I. DuPont de Nemours &    Co.-   Himilan: Ionomeric resins available from DuPont-Mitsui Polychemicals    Co., Ltd.-   Pandex: MDI-PTMG type thermoplastic polyurethanes available from DIC    Bayer Polymer.-   Polyisocyanate compound: 4,4′-Diphenylmethane diisocyanate.-   Thermoplastic elastomer: Available under the trade name “Hytrel    4001” from DuPont-Toray Co., Ltd.-   Titanium oxide: Available under the trade name-   “Tipaque R550” from Ishihara Sangyo Kaisha, Ltd.-   Polyethylene wax: Available under the trade name “Sanwax 161P” from    Sanyo Chemical Industries, Ltd.

Simultaneous with injection molding of the cover, numerous dimples wereformed on the surface of the cover, after which the cover wasspray-painted. In each example and comparative example, the dimples onthe surface of the ball after painting satisfied the parameters shown inTables 3 to 7 below. In these tables, the dimple types designated as Darefer to dimples having a diameter of 3.7 mm or more, and the dimpletypes designated as Db refer to dimples having a diameter of less than3.7 mm.

Here, referring to FIG. 2, the dimple depth (DP) in the tables is thevertical distance from a hypothetical flat plane L, traced by connectingthe positions where the dimple meets land areas, to the bottom (deepestposition) of the dimple. The dimple diameter (DM), as shown in FIG. 2,is the diameter (span) between positions where the dimple portion istangent with land areas (non-dimple-forming portions), i.e., between thehigh points of the dimple portion. In most cases, the golf ball has beensubjected to painting or the like. In such balls, the dimple diameterand depth refer to the diameter and depth after the coat of paint hasbeen applied.

With regard to the dimple patterns in the tables, the dimple patternsfor Examples 1 to 3 and for Comparative

Example 3 are shown in Table 3 (FIG. 3), the pattern for Example 4 isshown in Table 4 (FIG. 3), the pattern for Example 5 is shown in Table 5(FIG. 4), the patterns for Comparative Examples 1 and 5 are shown inTable 6 (FIG. 3), and the patterns for Comparative Examples 2 and 4 areshown in Table 7 (FIG. 5). These diagrams are all top views of the ball.In the respective examples, the bottom views of the ball have the samepattern as the top views, and are thus omitted.

TABLE 3 Examples 1 to 3, Number Comparative Example 3 of Diameter DepthVolume Dimple type dimples (mm) (mm) (mm³) Da-I 12 4.6 0.23 1.97 Da-II174 4.4 0.22 1.78 Da-III 18 4.0 0.21 1.40 Da-IV 6 3.8 0.20 1.21 Db-I 63.3 0.18 0.81 Db-II 36 2.4 0.13 0.32 Da-V 54 4.5 0.27 2.25 Db-III 6 3.40.16 0.75

TABLE 4 Number Example 4 of Diameter Depth Volume Dimple type dimples(mm) (mm) (mm³) Da-I 12 4.6 0.29 2.54 Da-II 174 4.4 0.28 2.26 Da-III 184.0 0.27 1.79 Da-IV 6 3.8 0.26 1.52 Db-I 6 3.3 0.22 0.98 Db-II 36 2.40.13 0.32 Da-V 54 4.5 0.33 2.78 Db-III 6 3.4 0.16 0.75

TABLE 5 Number Example 5 of Diameter Depth Volume Dimple type dimples(mm) (mm) (mm³) Da-I 40 4.0 0.24 1.88 Da-II 184 3.8 0.23 1.63 Db-I 963.2 0.22 1.07 Da-III 32 4.0 0.27 2.11 Da-IV 16 3.8 0.25 1.77 Db-II 163.1 0.20 0.91 Db-III 8 3.1 0.14 0.55

TABLE 6 Comparative Examples Number 1 and 5 of Diameter Depth VolumeDimple type dimples (mm) (mm) (mm³) Da-I 12 4.6 0.17 1.47 Da-II 174 4.40.17 1.35 Da-III 18 4.0 0.16 1.05 Da-IV 6 3.8 0.16 0.93 Db-I 6 3.3 0.150.66 Db-II 36 2.4 0.11 0.26 Da-V 54 4.5 0.21 1.76 Db-III 6 3.4 0.16 0.75

TABLE 7 Comparative Examples Number 2 and 4 of Diameter Depth VolumeDimple type dimples (mm) (mm) (mm³) Da-I 24 4.5 0.16 1.34 Da-II 150 4.30.16 1.22 Da-III 66 3.7 0.15 0.86 Db-I 18 2.7 0.12 0.36 Db-II 6 2.5 0.120.31 Da-IV 48 4.3 0.17 1.30 Da-V 12 3.8 0.16 0.96 Db-III 6 3.4 0.16 0.75Db-IV 6 3.3 0.15 0.66

Various properties of the resulting golf balls were investigated by thefollowing methods. The results are shown in Tables 8 and 9.

Deflection of Solid Core and Finished Ball

The solid core and the finished ball were placed on a hard plate, andthe deflection when compressed under a final load of 1,275 N (130 kgf)from an initial load state of 98 N (10 kgf) was measured.

Cover Material Hardness (Shore D Hardness)

The cover-forming material was formed under applied pressure to athickness of about 2 mm and the resulting sheet was held at 23° C. for 2weeks, following which the Shore D hardness of the sheet was measured inaccordance with ASTM D2240.

Ball Surface Hardness (Shore D Hardness)

Five finished balls were held isothermally at 23° C., following whichthe surface of each was measured at two randomly selected points in landareas without dimples. The measurements were carried out with a type Ddurometer in accordance with ASTM D-2240.

Initial Velocity

The initial velocity of the ball was measured using an initial velocitymeasuring apparatus of the same type as the USGA drum rotation-typeinitial velocity instrument approved by the R&A. The ball was heldisothermally in a 23±1° C. environment for at least 3 hours, then testedin a chamber at a room temperature of 23±2° C. The ball was hit using a250-pound (113.4 kg) head (striking mass) at an impact velocity of 143.8ft/s (43.83 m/s). One dozen balls were each hit four times. The timetaken for the ball to traverse a distance of 6.28 ft (1.91 m) wasmeasured and used to compute the initial velocity (m/s) of the ball.This cycle was carried out over a period of about 15 minutes.

Flight Performance

A driver manufactured by Bridgestone Sports Co., Ltd. (TOURSTAGE X-DRIVE701 (2009 model; loft angle, 9.5°)) was mounted on a swing robot, andthe distance traveled by the ball when hit at a head speed (HS) of 54m/s or 35 m/s was measured. The initial conditions here were set usingBridgestone Golf e5 balls (manufactured by Bridgestone Sports Co.,Ltd.), which have been sold in the United States since November 2009.When this ball was hit at a head speed of 54 m/s, the initial velocitywas set to 78.0±0.5 m/s, the launch angle was set to 9.7±0.5°, and theinitial backspin rate was set to 2,700±100 rpm. When the ball was hit ata head speed of 35 m/s, the initial velocity was set to 53.0±0.5 m/s,the launch angle was set to 14.0±0.5°, and the initial backspin rate wasset to 2,700±100 rpm.

TABLE 8 Example 1 2 3 4 5 Core Formulation A B C D D Diameter (mm) 37.337.3 37.3 37.3 37.3 Deflection (mm) 5.1 6.8 5.0 5.1 5.1 Inner Material(1) (1) (1) (1) (1) cover Material hardness (Shore D) 48 48 48 48 48layer Thickness (mm) 1.5 1.5 1.5 1.5 1.5 Outer Material (2) (2) (3) (2)(2) cover Material hardness (Shore D) 63 63 57 63 63 layer Thickness(mm) 1.2 1.2 1.2 1.2 1.2 Dimples Number of dimple types 8 types 8 types8 types 8 types 7 types Number of dimples 312 312 312 312 392 SR value(%) 74.4 74.4 74.4 74.4 71.6 VR value (%) 1.25 1.25 1.25 1.56 1.45 BallDiameter (mm) 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.3 45.3 45.3 45.345.3 Deflection (mm) 4.3 5.6 4.3 4.3 4.3 Surface hardness (Shore D) 6666 59 66 66 Initial velocity (m/s) 75.6 75.6 75.6 77.3 77.3 Formula: 4 ×VR + deflection 9.3 10.6 9.3 10.5 10.1 Flight HS Carry (y) 260.5 249.7258.0 248.0 251.2 54 m/s Total distance (y) 288.0 282.0 285.0 281.1283.5 HS Carry (y) 168.0 164.0 166.3 170.2 171.0 35 m/s Total distance(y) 194.0 191.0 192.0 198.0 196.8 Difference in total distance (y) 94.091.0 93.0 83.1 86.7 Total distance ratio HS54/HS35 1.48 1.48 1.48 1.421.44

TABLE 9 Comparative Example 1 2 3 4 5 Core Formulation A A E F ADiameter (mm) 37.3 37.3 37.3 37.3 37.3 Deflection (mm) 5.1 5.1 4.0 5.05.1 Inner Material (1) (1) (1) (1) (1) cover Material hardness (Shore D)48 48 48 48 48 layer Thickness (mm) 1.5 1.5 1.5 1.5 1.5 Outer Material(2) (2) (2) (4) (2) cover Material hardness (Shore D) 63 63 63 50 63layer Thickness (mm) 1.2 1.2 1.2 1.2 1.2 Dimples Number of dimple types8 types 9 types 8 types 9 types 8 types Number of dimples 312 336 312336 312 SR value (%) 73.4 76.0 74.4 76.0 73.4 VR value (%) 0.94 1.061.25 1.06 0.94 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 Weight (g)45.3 45.3 45.3 45.3 45.3 Deflection (mm) 4.3 4.3 3.4 4.3 4.3 Surfacehardness (Shore D) 66 66 66 53 66 Initial velocity (m/s) 75.6 75.6 75.677.3 75.6 Formula: 4 × VR + deflection 8.1 8.5 8.4 8.5 8.1 Flight HSCarry (y) 269.5 264.5 268.0 263.0 272.5 54 m/s Total distance (y) 291.1288.1 292.5 289.0 294.1 HS Carry (y) 166.6 163.6 170.5 168.0 168.4 35m/s Total distance (y) 190.0 188.0 195.5 191.6 192.3 Difference in totaldistance (y) 101.1 100.1 97.0 97.4 101.8 Total distance ratio HS54/HS351.53 1.53 1.50 1.51 1.53

From the results in Tables 8 and 9, it was confirmed that, compared withthe golf balls in Comparative Examples 1 to 5, the golf balls inExamples 1 to 5 of the present invention had a decrease in distance onshots taken at low head speed that was suppressed relative to the largereduction in distance on shots taken at a high head speed. That is, theworking examples of the invention were confirmed to be golf balls whichhad a small difference between the distance at high head speed and thedistance at low head speed, and were thus able to achieve a superiordistance in the low HS range while suppressing the distance in the highHS range. The results obtained for the golf balls in ComparativeExamples 1 to 5 were as follows.

In Comparative Example 1, although the initial velocity was held down,thus keeping the total distance on shots taken at a head speed of 54 m/sat or below about 290 yards, because the dimple volume ratio was small,the total distance on shots taken at a head speed of 35 m/s alsodecreased significantly, resulting in a large total distance ratioHS54/HS35.

In Comparative Example 2, although the initial velocity was held down,thus keeping the total distance on shots taken at a head speed of 54 m/sat or below about 290 yards, because the dimple volume ratio was small,the total distance on shots taken at a head speed of 35 m/s alsodecreased significantly, resulting in a large total distance ratioHS54/HS35.

In Comparative Example 3, because the ball had a small deflection, thetotal distance traveled by the ball when struck at a head speed of 54m/s exceeded 290 yards.

In Comparative Example 4, although the total distance on shots taken ata head speed of 54 m/s was held to below 290 yards, because the ball hada low surface hardness, the total distance on shots taken at a headspeed of 35 m/s also decreased significantly, resulting in a large totaldistance ratio HS54/HS35.

In Comparative Example 5, because the dimple volume ratio was small, thetotal distance on shots taken at a head speed of 54 m/s exceeded 290yards, in addition to which the total distance ratio HS54/HS35 waslarge.

1. A golf ball comprising a solid core, a cover of at least one layer,and a plurality of dimples on a surface of an outermost layer of thecover, wherein the dimples number at least 250 but not more than 500 andhave a volume ratio (VR) of from 1.20 to 1.60%, the outermost coverlayer has a surface hardness, expressed as the Shore D hardness, of from58 to 75, and the ball has a deflection, when compressed under a finalload of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf),of from 4.0 to 6.0 mm.
 2. The golf ball of claim 1, wherein the ball,under the conditions of a head speed of 54 m/s, a ball initial velocityof 78.0±0.5 m/s, a launch angle of 9.7±0.5° and an initial backspin rateof 2,700±100 rpm, has a total distance of not more than 290 yards. 3.The golf ball of claim 1, wherein the ball has a ratio of total distancetraveled when struck at a head speed of 54 m/s to total distancetraveled when struck at a head speed of 35 m/s (HS54/HS35) of from 1.30to 1.50.
 4. The golf ball of claim 1, wherein the dimple volume ratioand the deflection satisfy the relationship expressed by the formula:4×VR+deflection=9.0 to 11.0.